Electric arc furnace

PRESENTATION ON:ELECTRIC ARC FURNACE

Gulfam Hussain

ELECTRIC ARC FURNACEELECTRIC ARC FURNACE: An Electric Arc Furnace (EAF) is a

furnace that heats charged material by means of an electric arc.

Arc furnaces range in size from small units of approximately one ton capacity (used in foundries for producing cast iron products) up to about 400 ton units used for secondary steelmaking

TEMPERATURE RANGE Industrial electric arc furnace

temperatures can be up to 1,800 °C, (3272 °F) while laboratory units can exceed 3,000 °C. (5432 °F)

Arc furnaces differ from induction furnaces in that the charge material is directly exposed to an electric arc, and the current in the furnace terminals passes through the charged material.

HISTORY The first electric arc furnaces were

developed by Paul Héroult, of France, with a commercial plant established in the United States in 1907.

The Sanderson brothers formed The Sanderson Brothers steel Co. in Syracuse, New York, installing the first electric arc furnace in the U.S.

This furnace is now on display at Station Square, Pittsburgh, Pennsylvania.

TYPES OF EAF Two kinds of electric current may be

used in Electric Arc Furnaces: direct (DC) EAF alternating (AC) EAF Three-phase AC Electric Arc Furnaces

with graphite electrodes are commonly used in steel making.

CONSTRUCTION The furnace consists of a spherical hearth

(bottom), cylindrical shell and a swinging water-cooled dome-shaped roof.

The roof has three holes for consumable graphite electrodes held by a clamping mechanism.

The mechanism provides independent lifting and lowering of each electrode.

The water-cooled electrode holders serve also as contacts for transmitting electric current supplied by water-cooled cables (tubes).

The electrode and the scrap form the star connection of three-phase current, in which the scrap is common junction.

The furnace is mounted on a tilting mechanism for tapping the molten steel through a tap hole with a pour spout located on the back side of the shell.

The charge door, through which the slag components and alloying additives are charged, is located on the front side of the furnace shell.

The charge door is also used for removing the slag (de-slagging).

REFRACTORY LINING OF AN EAF

Refractory linings of Electric Arc Furnaces are made generally of resin-bonded magnesia-carbon bricks.

When the bricks are heated the bonding material is coked and turns into a carbon network binding the refractory grains,preventing wetting by the slag and protecting the lining the from erosion and chemical attack of the molten metal and slag.

OPERATION/WORKING The scrap is charged commonly from

the furnace top. The roof with the electrodes is swung

aside before the scrap charging. The scrap arranged in the charge

basket is transferred to the furnace by a crane and then dropped into the shell.

Lower voltages are selected for this first part of the operation to protect the roof and walls from excessive heat and damage from the arcs.

Once the electrodes have reached the heavy melt at the base of the furnace and the arcs are shielded by the scrap,

the voltage can be increased and the electrodes raised slightly, lengthening the arcs and increasing power to the melt.

CHEMICAL AND PHYSICAL PROCESSES IN AN EAF

Melting Melting process starts at low voltage (short

arc) between the electrodes and the scrap. The arc during this period is unstable. In order to improve the arc stability small

pieces of the scrap are placed in the upper layer of the charge.

The electrodes descend melting the charge and penetrating into the scrap forming bores.

The molten metal flows down to the furnace bottom.

When the electrodes reach the liquid bath the arc becomes stable and the voltage may be increased (long arc).

The electrodes are lifting together with the melt level. Most of scrap (85%) melt during this period.

Temperature of the arc reaches 6300ºF (3500ºC).

OXIDIZING STAGE

At this stage excessive carbon, phosphorous, silicon and manganese oxidize.The process is similar to that in Basic Oxygen Furnace.

Basic oxidizing slag composed of lime (CaO) and ion ore (FeO) is used during the oxidizing period.Gaseous oxygen may be blown into the melt for additional oxidizing.

REDUCING STAGE New slag composed mainly of lime (CaO), CaF2 (as slag fluidizer) is

added at this stage for formation of basic reducing conditions. The function of this slag is refining of the steel from sulfur and absorption of oxides, formed as a result of

deoxidation (”killing”). The excessive oxygen dissolved in the melt during oxidizing period is removed by metallic deoxidizersMn, Si, Al:

[Mn] + [O] = (MnO)

[Si] + 2[O] = (SiO2)

2[Al] + 3[O] = (Al2O3)

Basic reducing slag is favorable for desulfurization in accordance to the reaction:

[S] + (CaO) = (CaS) + [O]

Oxide and sulfide non-metallic inclusions are absorbed by the slag.

Alloying elements (Cr, Ni, Mo, V, etc.) are added after deoxidation.

In many cases the processes of “killing” (deoxidation), desulfurization, alloying and final heating are performed outside of the furnace - Ladle refining

Iron oxide causes increase of Oxygen content in the molten steel according to the reaction:(square brackets [ ] - signify solution in steel, round brackets ( ) - in slag, curly brackets {} - in gas)

(FeO) = [Fe] + [O]

Oxygen dissolved in the melt oxidizes carbon, phosphorous, silicon and manganese:

[C] + [O] = {CO}

[Si] + {O2} = (SiO2)

[Mn] + 1/2{O2} = (MnO)

2[P] + 5/2{O2} = (P2O5)

Carbon monoxide partially burns in the atmosphere:

{CO} + {O2} = {CO2}

The formed oxides are absorbed by the slag. CO bubbles floating up through the melt result in refining of the steel from non-metallic inclusions and hydrogen removal.

Gaseous products CO and CO2 are removed by the exhausting system. Oxidizing potential of the atmosphere is characterized by the post-combustion ratio: {CO2}/({CO2}+{CO}).

The oxidizing slag enriched with phosphorous and other oxides formed during this period is removed from the furnace to a slag pot (de-slagging).

ADVANTAGES OF ELECTRIC ARC FURNACE The use of EAFs allows steel to be

made from a 100% scrap metal feedstock.

This greatly reduces the energy required to make steel when compared with primary steelmaking from ores

. Another benefit is flexibility: while blast furnaces cannot vary their production by much and can remain in operation for years at a time,

EAFs can be rapidly started and stopped, allowing the steel mill to vary production according to demand.

During the peak of global financial meltdown in 2009, an estimated quantity of only 1 million tonne was produced in USA employing EAF technique.

Although steelmaking arc furnaces generally use scrap steel as their primary feedstock,

if hot metal from a blast furnace or direct-reduced iron is available economically, these can also be used as furnace feed.

A typical steelmaking arc furnace is the source of steel for a mini-mill, which may make bars or strip product. Mini-mills can be sited relatively near to the markets for steel products, and the transport requirements are less than for an integrated mill, which would commonly be sited near a harbour for access to shipping.

ENVIRONMENTAL ISSUES Although the modern electric arc furnace is a highly efficient recycler of

steel scrap, operation of an arc furnace shop can have adverse environmental effects. Much of the capital cost of a new installation will be devoted to systems intended to reduce these effects, which include:

Enclosures to reduce high sound levels Dust collector for furnace off-gas Slag production Cooling water demand Heavy truck traffic for scrap, materials handling, and product Environmental effects of electricity generation

Because of the very dynamic quality of the arc furnace load, power systems may require technical measures to maintain the quality of power for other customers; flicker and harmonic distortion are common side-effects of arc furnace operation on a power system.